The present invention concerns a method of measuring the optical density of an original photographic image, and apparatus for carrying out the method, to determine how much printing light of each color penetrates the original when making photographic prints therefrom.
A method and apparatus of this type are described in the German Patent Publication No. OS 3,737,775. Measuring light travels through an original (either a positive or negative) and is resolved into at least one color spectrum by means of a spectroscope. The intensities of the light in the various ranges of wavelength are measured separately. Each result is multiplied by a factor .THETA..sub.x.sup.BGR that characterizes the spectral sensitivity of the color print medium at that range to one of the colors red, green, and blue. The totals of the weighted results for each color are employed to determine how much printing light the apparatus will use. Weighing the results for the separate ranges of the spectrum with wavelength-dependent factors theoretically makes it possible to simulate any filter transmission curve. The curves can accordingly be adapted very precisely to the sensitivity of the paper.
The disclosed method, however, does have certain limitations. First, since the light traveling through any area of the original being sensed must be distributed among a large number of light sensitive elements, specifically the pixels of a CCD, each pixel receives only a small amount of light. Secondly, the method demands a relatively long series of calculations, specifically separate multiplication and addition for each individually measured range of the spectrum, to attain corrected brightness levels for each of the three colors at each area of the original.